A number of recent reports on the presence of smooth muscle in ovarian follicles of various mammals have rekindled controversy concerning the possible role of follicular smooth musculature in mammalian ovulation. No ultrastructural model for the function of these cells in ovulation has yet been presented. The earlier observation that intrafollicular fluid pressure does not rise just prior to ovulation suggests that the mammalian follicle is not being squeezed during the process. However, we have good evidence for a functional contractile system in ovulating follicles of fish and amphibians. Mammalian follicles are unusual in that the contained oocyte is relatively small; it does not fill the follicle. We suggest that the decreased size of the gamete without a commensurate reduction in follicle size, has made smooth muscle-like components no longer required to accomplish oocyte extrusion. We postulate that, for oocyte extrusion from a large follicle which is completely filled by its gamete, a contractile system is necessary. To examine this proposition, we will investigate the structure of the ovarian follicles of reptiles, to determine whether this amniote group also possesses a follicular contractile system. No relevant structure information is available from the literature. We will then attempt to relate ovulation patterns in reptiles to our previous observations on anamniotes, and compare them with available information on mammals. In addition, we propose to study the physiology of the contractile system of the amphibian and teleost follicle. We will attempt to define the conditions under which the amphibian follicle wall will contract in vitro, with attention to the timing of contraction relative to the other major element of the ovulatory response, apical rupture formation. We will also study the development of intrafollicular pressure during amphibian ovulation using an electronic method.